Dr Krause said symptoms include severe headache, neck stiffness, high fevers and seizures, and in some cases there is a progression to delirium, coma and possibly death.

The areas most at risk at present are the Victoria River region, and the Barkly and Central Australian regions in Northern Territory. In West Australia, the affected areas are the Gascoyne, Murchison, Kimberley and Pilbara regions.

"Above average wet season rains and associated flooding in these regions has enabled the breeding of a large number of the types of mosquitoes that can transmit the virus," Dr Krause said.

There is no vaccine for the disease and the only way to prevent it is by avoiding being bitten.

Dr Krause warned campers and travellers in particular to take particular care and avoid being outside between sunset and mid-evening, when mosquitoes are active.

Health authorities raised the alarm when a number of chickens in a sentinel chicken warning system, put in place throughout the top end of Australia, became ill, indicating that the virus was active and widespread.